A sewing unit with a feed device for a fabric holder has been known from DE 36 07 107 C1. The feed device according to the exemplary embodiment shown in FIG. 3 of this patent has guides for accommodating support elements for guide rod mechanisms acting on the fabric holder, one of which is designed as a parallel guide rod mechanism, and another as a single guide rod mechanism. Each support element is connected to a stationary drive for movement along its guide.
If the guides are arranged nearly at right angles corresponding to the representations in FIG. 3 of the patent, the movement of the fabric holder, which must take place in the direction of extension of a guide, must be brought about by the drive that is associated with the support element that is displaceable on the guide alone, while the drive of the support element arranged on the other guide only guides a compensating movement toward the fabric holder. This leads to a high load of the first-named drive, on the one hand, and, on the other hand, to an unfavorable dynamic behavior of the feed device, especially at high stitch frequency, because the stitch-by-stitch acceleration as well as the deceleration of all movable parts must be ensured by a single drive.
In the above-described arrangement of the two guides in relation to one another, the range of movement of the fabric holder is influenced not only by the length of the guides, but by a plurality of other parameters as well, e.g., the length of the guide rod or the angle through which the guide rods are able to move. The range of movement of the fabric holder cannot consequently be extended as desired by extending the guides, but it can be extended only within the framework of the limits defined by the other elements of the feed device. Thus, even though such a feed device is suitable for moving a fabric to be sewn through the stitch formation zone of the sewing machine, it fails when the fabric to be sewn is additionally to be brought to the sewing machine from a remote insertion or preparation station or to a likewise remote removal or further processing station.
EP 0 361 640 A1 discloses a sewing unit with a feed device, which has a carriage, which is displaceable on rails and is provided with guides extending at right angles to the rails, on which guides support elements for arms acting on halves of the fabric holder are displaceably arranged. For feeding the carriage along the rails, the carriage is connected to a stationary drive. The movement of the support elements along the guides is brought about by a second drive carried by the carriage.
A fabric to be sewn can be transported by the feed device from an insertion station to the sewing machine and can be moved along the stitch formation site there. However, the carriage must be accelerated or decelerated together with the second drive arranged thereon during movements at right angles to the direction of extension of the carriage. Due to the relatively large mass of the carriage, which is caused by this drive, the feed device possesses poor dynamic properties. It is therefore unsuitable for use on modern sewing units which operate at very high stitch frequency during feed.